Present day electronic private automatic branch exchanges (PABX's) are bulky, costly and consume considerable power. These shortcomings can be reduced dramatically by implementing the PABX in the latest VLSI technology. Recently, a single chip PABX which can provide voice and data service to 40 subscribers and that can be interconnected to form a larger PABX system has been developed. The single chip PABX is described by M. Cooperman et al in "Experimental Single Chip PABX", IEEE Journal of Solid State Circuits, Vol. SC-21, No. 2, April 1986, pp. 247-251 and in U.S. Pat. No. 4,630,284 assigned to the assignee of the present invention. A single chip PABX system utilizes digital communication for sending both voice and data between a central switch, the local single chip PABX, telephones and data terminals. The connection between each telephone and the local PABX utilizes twisted pair wiring operating in the time division duplex mode.
Although VLSI circuits can perform highly complex functions with extremely small geometry, they are very limited in power dissipation. Conventional digital line driving techniques employ terminations at the receiving end to prevent reflections. The line driver transistor geometry must be large, and the driver must deliver several hundred milliwatts to the termination load. In order to integrate the line driving and line receiving functions into the single chip PABX, transmission lines that are unterminated at the receiving end are utilized. As a result of the interaction between the transmitted and reflected waveforms, power is dissipated only during logic transitions. The power averages 10 milliwatts per line driver for a data rate of 310 kilobits per second and a maximum line length of 400 feet.
The single chip PABX system utilizes a communication protocol wherein a message is transmitted from the PABX chip to the telephone during the first portion of a frame and a response is transmitted from the telephone to the PABX chip during a second portion of the frame. Transmission distances are relatively short so that messages and responses can be detected during the prescribed time intervals. A response is transmitted at the end of a message and is received within one bit time by the PABX chip. Similarly, a message is transmitted following the end of the response and is received within one bit time by the telephone. As a result, the complexity and power requirements of asynchronous receivers are eliminated. Nonetheless, it is possible for the PABX chip and the telephone to lose synchronization, for example, due to noise on the transmission line. The loss of synchronization is not a serious problem if synchronization can be reestablished in a short time. It is desirable to provide a method for reestablishing synchronization without additional hardware or added complexity.
It is a general object of the present invention to provide improved methods for data communication.
It is another object of the present invention to provide methods for reestablishing synchronization between two subsystems connected by a data communication link when synchronization is lost.
It is yet another object of the present invention to provide a digital communication link capable of operation in the presence of electrical noise.
It is still another object of the present invention to provide a telecommunication system adapted for integration on one or a few integrated circuit chips.